Selective catalytic reduction (SCR) is commonly used to remove NOx (i.e., oxides of nitrogen) from the exhaust gas produced by internal combustion engines, such as diesel or other lean burning (gasoline) engines. In such systems, NOx may be continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to the exhaust gas encountering an SCR catalyst that is capable of achieving a high conversion of NOx.
According to certain systems, the reductant may be introduced into the exhaust gas by controlled injection, such as, for example, the controlled injection of gaseous ammonia, aqueous ammonia, or aqueous urea. Aqueous urea that is dosed into the exhaust stream may hydrolyses to gaseous ammonia. Alternatively, gaseous ammonia may be delivered into the exhaust gas stream through the use of an ammonia storage and delivery system, which utilizes high density storage to accommodate ammonia in solid form. With the different types of systems for introducing reductant into the exhaust gas, the SCR catalyst, which is positioned in the exhaust gas stream, causes a reaction between NOx present in the exhaust gas and a NOx reducing agent (e.g., ammonia) to convert the NOx into nitrogen and water.
Proper operation of the SCR system involves precise control of the amount (i.e., dosing level) of ammonia (or other reductant) that is injected into the exhaust gas stream. For example, injection of too much reductant may cause a slip of ammonia in the exhaust gas, whereas injection of too little reductant may cause a less than optimal conversion of NOx.
SCR systems often utilize NOx sensors in order to determine proper reductant dosing levels. For example, a NOx sensor can be positioned in the exhaust stream between the engine and the SCR catalyst for estimating or detecting the content of NOx that is in the exhaust gas that is being emitted from the engine, also referred to as engine-out NOx level. Such a NOx sensor is commonly referred to as an engine-out NOx sensor or an upstream NOx sensor. An engine control unit can use the output from the engine-out NOx sensor to determine the amount of reductant that should to be injected into the exhaust stream.
Commercially available NOx sensors are expensive and have other operational drawbacks. For example, NOx sensors may have a measuring range of 100-1500 parts per million with an accuracy of plus or minus 15%. Additionally, the accuracy of NOx sensors can be affected by environmental and/or operating conditions such as dew point, system voltage, and oxygen concentration, among other drawbacks. In this regard, some NOx sensors only work properly when the exhaust gas is above a threshold temperature, which may be on the order of 125° C.-130° C. As a result, NOx sensors may not suitable for determining dosing levels during certain engine operating conditions, such as during low idle conditions or engine warm-up. Additionally, the inclusion of NOx sensors, in addition to other sensors used in connection with On-Board Diagnostics requirements, may translate into additional sensor implementation costs. Further, efforts are typically required to avoid NOx sensors from being positioned where the electronics of the NOx sensors may be exposed to high exhaust gas temperatures and dew point exposure, which may otherwise translate into increased warranty and maintenance costs.